Zinc ferrite crystallizes in the spinel lattice and may be used as starting material for soft magnets or as corrosion protective or color pigment, depending on its stoichiometric composition, the additives contained in it, its particle sizes, its crystal form and its surface properties.
Non-ferrimagnetic color pigments have become known in the English language world as "tan" pigments.
The preparation of zinc ferrite pigments is described in US-A 3 832 455. In the process described there, ferric hydroxide is precipitated from iron-II sulphate solution on zinc oxide or zinc carbonate at pH values from 5 to 6 and temperatures from 49.degree. to 52.degree. C., the resulting suspension is filtered and the solid residue is washed, dried and annealed.
In the process according to US-A 2 904 395, the zinc ferrite pigments are prepared both by mutual precipitation from the corresponding solutions containing iron and zinc, followed by filtration, washing, drying and annealing, and by annealing an intimate mixture of ferric hydroxide and zinc oxide which has been obtained in aqueous suspension. Calcining is carried out at temperatures of up to 1000.degree. C. with the addition of catalysts such as hydrochloric acid or zinc chloride.
It is known from US-A 4 222 790 that the annealing process for the preparation of zinc ferrite or magnesium ferrite may be improved by the addition of an alkali metal silicate to the mixture. Aluminium sulphate is added as flocculating agent to improve filtration.
The addition of compounds forming A1.sub.2 O.sub.3 and P.sub.2 O.sub.5 for the calcination of color pure, chloride free zinc ferrite pigments is described in DE-A 3 136 279.
According to EP-B 154 919, color pure zinc ferrite pigments may be obtained without additives by using needle shaped .alpha.-FeOOH of a certain particle size and surface and zinc oxide of a certain surface.
Boric acid or boron phosphate is added after annealing to improve the cooling process.
Lastly, according to JP-B 570 11 829, titanium dioxide is added to produce heat resistant yellow zinc ferrite color pigments.
The properties of the anisometric zinc ferrites obtained are described in detail in T.C. Patton, Pigment Handbook, Volume 1, Properties and Economics, pages 347 and 348, John Wiley & Sons, New York, 1973.
These anisometric zinc ferrites are generally distinguished by excellent resistance to light and weathering and high thermostability. They are therefore also used instead of the less thermostable mixtures of iron yellow and iron oxide red.
They have also been found useful in particular for colouring sand granulates, lime sandstone, enamels, ceramic glazes, stoving lacquers and plastics.
Although zinc ferrite pigments are prepared at high temperatures, above 700.degree. C., they are subject to varying degrees of color changes in some systems, especially when they are used for coloring organic materials, and therefore in many cases can no longer be regarded as thermostable in these systems. Thus, for example, when these pigments are used for coloring plastics, a noticeable color shift to darker, more dirty colors takes place at temperatures of about 250.degree. C. and upwards so that zinc ferrite pigments are not suitable for coloring these substances. This applies especially to the coloring of plastics which require relatively high processing temperatures, such as polyamides or ABS plastics.
It is an obJect of the present invention to provide zinc ferrite color pigments which do not have the above described disadvantages of the state of the art.